This invention relates generally to devices for general circuit protection including electrical distribution and motor control applications, more particularly to simple, reusable, potentially low cost devices that can be tailored to a plurality of applications and most particularly to current limiting devices for relatively high power applications where the system voltage is equal to or greater than 100 V and the short-circuit current is equal to or greater than 100A utilizing an electrically conductive composite material and an inhomogeneous distribution of resistance structure.
There are numerous devices that are capable of limiting the current in a circuit when a short-circuit occurs. One current limiting device presently being used includes a filled polymer material which exhibits what is commonly referred to as a PTCR (positive-temperature coefficient of resistance) or PTC effect. The unique attribute of the PTCR or PTC effect is that at a certain switch temperature the PTCR material undergoes a transformation from a more conducting material to a more resistive material. In some of these prior current limiting devices, the PTCR material (typically polyethylene loaded with carbon black) is placed between pressure contact electrodes.
In operation, these prior current limiting devices are placed in the circuit to be protected. Under normal circuit conditions, the current limiting device is in a highly conducting state. When a short-circuit occurs, the PTCR material heats up through resistive heating until the temperature is above the switch temperature. At this point, the PTCR material resistance changes to a high resistance state and the short-circuit current is limited. When the short-circuit is cleared, the current limiting device cools down to below the switch temperature and returns to the highly conducting state. In the highly conducting state, the current limiting device is again capable of switching to the high resistance state in response to future short-circuit events.
U.S. Pat. No. 5,382,938 describes a PTC element comprising a body of an electrically conductive polymer composition having a resistivity with a positive temperature coefficient, the body defining two parallel end surfaces and two electrodes arranged in contact with the end surfaces for carrying current through the body. The polymer composition of the body includes a polymer material and an electrically conductive powdered material distributed in the polymer material. The expression PTC element is the accepted term for an element which exhibits a positive temperature coefficient of resistance with a switch temperature as shown in FIG. 1 of U.S. Pat. No. 5,382,938. At least one of the parallel surfaces on the body is in free contact with an electrode or with a parallel surface on another body of electrically conductive polymer composition. A pressure device inserts a pressure directed perpendicularly to the parallel surfaces on the body, or the bodies, on the electrodes. The pressure device is preferably provided with a pressure-exerting devices with the ability to be resilient. After changing from a low resistance to a high resistance state, the PTC element returns to the initial resistance and is reusable after having been subjected to short-circuit currents. The parallel surfaces on the body, or the bodies, of polymer composition may be concentric. PTC elements are used in electric circuits as overcurrent protection.
U.S. Pat. No. 5,313,184 describes an electric resistor having a resistor body arranged between two contact terminals. The resistor core includes an element with PTC behavior, which, below a material-specific temperature, forms an electrically conducting path running between the two contact terminals. The resistor can be simple and inexpensive, but still having high rate current-carrying capacity protected against local and overall overvoltages. This is achieved by the resistor core additionally containing a material having varistor behavior. The varistor material is connected in parallel with at least one subsection of the electrically conducting path, forming at least one varistor, and is brought into intimate electrical contact with the part of the PTC material forming the at least one subsection. The parallel connection of the element with PTC behavior and the varistor can be realized both by a microscopic construction and by a macroscopic arrangement.
European Patent 0,640,995 A1 describes an electrical resistance element containing a resistive material that has PTC characteristics and is arranged between two plane-parallel electrodes that are subjected to pressure, whereby the resistive material consists of a polymer matrix and two filler components that consist of electrically conducting particles, wherein the two filler components are embedded in the polymer matrix. In the event of a short-circuit current, the resistivity of the resistive material changes, in a step-like manner above a limiting temperature value, in a surface layer that lies on the electrodes and that contain at least the first of the two filler components. The second of the two filler components is selected in such a way that a composite material that contains at least a polymer matrix and the second filler component exhibits PTC characteristics with a step characteristic that is higher by at least one order of magnitude, relative to the surface layer. At the same time, this composite material has a resistivity that is lower, by at least one order of magnitude, than a composite material that is formed from the polymer matrix and the first filler component.
Despite the efforts described above, all of which involve PTC behavior, to provide a simpler, more durable current limiting devices, a need still exists for more even simpler and more durable, reusable potentially low cost current limiting devices for general circuit protection in electrical distribution and motor control applications that can be tailored to a plurality of applications. Such devices should protect the circuits for at least more than for one short circuit and preferably many short circuits, not require that the material exhibit PTCR effect and be usable with high voltages of 100 to 500 volts or more.